hed. This is the job of the blood, with the heart and vessels acting asfacilitators. After depositing the necessary nutrients with each of the cells, theblood returns to the heart, carrying with it waste products it has picked upalong the way. These are eliminated through a filtering process in the lungsand kidneys. By now the supply of oxygen within the blood is nearly exhausted, and it istime to restock its supplies and begin the journey again. Incredibly, the wholeprocess has taken just 20 seconds. During the course of an average life, the heart pumps tens of millions ofgallons of blood. It is estimated that the amount would easily fill a24-foot-wide cylinder to a height greater than the Empire StateBuilding. Perhaps more impressively, the circulatory system has the computer-likeability to direct greater and lesser amounts of blood to various areas of thebody according to their immediate needs. This explains why athletes oftenforego eating just prior to a match. During the process of digestion, thegastric organs require more blood to complete their work. As if that weren'tenough, the heart is also wired through the nervous system to respond to alarge variety of physical and emotional stimuli. Witness the quickening of theheart at the touch of a loved one. What Can Go Wrong Most circulatory problems are caused by a blockage in an artery, which isknown as atherosclerosis or hardening of the arteries. No one knows for surewhy this happens, but the prevailing theory states that something causes theprotective inner lining of an artery wall to be injured. Once impaired, thecollagen in the lining is exposed. That, in turn, attracts platelets and toxicsubstances from the bloodstream, which enter the artery wall. Eventually, theprocess leads to a buildup of debris, narrowing of the artery and finally,blockage.Interestingly enough, although the heart has a continual flow of bloodstreaming in and out of its chambers, it is unable to take the no...
